Proximity sensor

ABSTRACT

An energy curtain is formed by exciting high order beam modes in an open resonant cavity. Depending on mode number, the energy resonates in a mathematically determined pattern. A detector senses changes in the resonant condition caused by the presence of an object in the pattern. The apparatus may be used in a variety of applications, including the protection of a machine operator from a hazardous area of a machine, parts counting, and intrusion detection.

This is a division, of application Ser. No. 534,997, filed Dec. 20,1974, abandoned.

CROSS-REFERENCE TO CO-PENDING APPLICATIONS

Reference is made to my two co-pending applications, entitled "HighOrder Beam Mode Resonator" now U.S. Pat. No. 3,979,695 and "AcousticResonant Cavity" now U.S. Pat. No. 3,948,350, which were filed on evendata with this application, and which are assigned to the same assigneeas this application.

BACKGROUND OF THE INVENTION

The present invention relates to a proximity sensor, which is a devicewhich reacts to the presence of an object within a certain spatialvolume, without direct physical contact between the proximity sensor andthe object. The basic components of the proximity sensor are an energysource and a sensor. Interaction between the object and the energycauses a change in some aspect of the sensor output (amplitude,frequency, etc.) from which the presence of the object is inferred.

Among the applications for a proximity sensor are the fields ofinstrusion detection, the counting of parts produced by a productiontool, and the protection of a machine operator from the hazardous areaof a machine. Increasingly severe requirements, due to expandingtechnology and more alien environments, have necessitated research toimprove performance parameters of present proximity sensors, as well asthe development of new approaches.

An area of particular recent concern has been the need for a device as aguard or shutoff control for machine tools. The purpose of the devicewould be to deactuate a machine should the operator insert his handsinto an area which is dangerous. For example, such an apparatus isneeded for a punch, forging, or stamping press where it is desirable tocause the press to shut down at once and discontinue operation shouldthe operator's hands be in the press area during operation.

A number of safety devices for machine tools have been proposedrecently. Among the types of safety devices proposed are the lightcurtain type, the capacitance type, and the physical barrier type.

The light curtain type of safety device utilizes a light beam or beamswhich form a curtain of light. This may be achieved by the sequentialoperation of a plurality of light sources, as shown in U.S. Pat. No.3,746,863, or by the scanning of a single light beam and the reflectionof the scanned beam between a pair of spaced reflectors. When the lightbeam is interrupted so that it does not reach a detector, the machinetool is deactuated. One problem with these systems is that they arerelatively complex. In the case of the light curtain of U.S. Pat. No.3,746,863, a large number of light sources and detectors are required aswell as circuitry for the sequential operation of these sources anddetectors. In the case of the scanning light beam, only a single lightsource and detector are used, but moving parts, including a scanningmirror, are required.

The capacitance type of safety device uses an antenna near the dangerousarea. The presence of an object near the antenna causes a change of theloading on the antenna which is then sensed. One problem with this typeof sensor is that it will sense movement three feet or more away fromthe antenna, i.e. the space which it monitors is not well defined.

The physical barrier type of safety device utilizes a screen or platewhich is interposed between the operator and the dangerous area. Themachine will operate only when the plate is in position. In someapplications, the operator must move the plate in order to load themachine and then replace the plate in order to start the machine. Thiscauses additional operations to be performed by the operator, therebyreducing productivity. In other applications the physical barrier ismoved automatically, but this is still a time consuming motion. In bothcases the physical barrier obstructs the operator's view of the die.

SUMMARY OF THE INVENTION

The proximity sensor of the present invention utilizes energy which isresonating in a high order beam mode in a predetermined pattern. Achange in the resonant condition caused by the presence of an object inthe predetermined pattern is detected.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a reasonant cavity of the type used in the presentinvention.

FIGS. 2a and 2b show a front and a cross-sectional side view,respectively, of a rectangular reflector for use in one embodiment ofthe present invention.

FIGS. 3a and 3b shows the energy distribution of a resonant rectangularTEM₈₀ mode formed with rectangular reflectors of the type shown in FIGS.2a and 2b.

FIGS. 4a and 4b show the apparatus of the present invention used as adevice for machine operator safety.

FIGS. 5a and 5b show front and side views, respectively, of a circularreflector for producing cylindrical resonant modes for use in thepresent invention.

FIG. 6 shows the energy distribution of a resonant cylindrical TEM₀₈mode.

FIG. 7 shows a partial cross-sectional view of machine operator safetyapparatus utilizing a cylindrical resonant mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a proximity sensor which utilizes perturbationof a resonant beam mode as the sensing mechanism. The beam moderesonator is chosen to resonate in a mode whose geometry coincides witha desired shape of an energy curtain. The presence of an object isinferred from its perturbing effects on resonant parameters of thecavity.

Beam modes are so named because they are mathematically identical to thepossible cross-sectional power levels of a laser beam. (H. Kogelnik andT. Li, "Laser Beams and Resonators," Applied Optics, 5, 1550-1567(October, 1966)), or the so-called beam wave guide. (G. Gouban and F.Schwering, "On the Guided Propagation of Electro-magnetic Wave Beams,"IRE Trans. on Antennas and Propagation, AP-9, 248-256 (May, 1961)). Inthese well-known technologies, however, the goal is to suppress highorder modes, since they have a greater spatial extent and hence greaterloss. Although observation of modes as high as TEM₀₇ have been reportedin the laser literature, they are not usually welcome in laser or beamwave-guide technology. In the applications of this invention, however,it is precisely the greater spatial extent of the higher order modeswhich is exploited. Indeed, in the cylindrical annular case, modes ashigh as TEM₀₋₂₅ have been used at a wavelength of 3 cm. For shorterwavelengths even higher order modes are used.

FIG. 1 shows a diagramatic representation of the present invention. Theresonant cavity is formed by reflectors 10 and 12 which are two concavecurved surfaces facing one another and separated from one another alongan axis. Energy source 14 provides energy into the cavity formed byreflectors 10 and 12. As shown in FIG. 1, the energy may be supplied tothe resonant cavity through a hole in one of the reflectors. Detector 16senses the energy in the resonant cavity.

The spacing and shape of reflectors 10 and 12 and the wavelength of theenergy supplied by energy source 14 will determine the particularspatial distribution of the resonant energy in the resonant cavity. Theintrusion of a foreign object, such as a machine operator's hand, into aresonant cavity destroys the resonance because the foreign objectabsorbs energy, scatters energy out of the resonant cavity, and causes ashift in the resonant frequency. This disturbance in the resonance issensed by detector 16, which can be used to shut off a machine, to soundan alarm, or for a variety of other purposes.

Many design alternatives are available for the present invention. Forexample, both electromagnetic energy (in the microwave region of thespectrum) and acoustic energy have been used in the present invention.The preferred wavelength of the energy is between about 0.1 mm and about10 cm. Acoustic energy has one important advantage over electromagneticenergy. A given wavelength can be produced with acoustic waves at a muchlower frequency than with microwaves, due to the lower speed ofpropagation of acoustic waves. The production of resonant beam modeswith acoustic waves is described in my previously mentioned patentapplication entitled "Acoustic Resonant Cavity".

As shown in FIG. 1, the reflectors 10 and 12 have curved surfaces.Although plane reflectors can also be used, experiments have shown thatreflector alignment becomes very critical, which is a disadvantage formost applications. When spherical surfaces are used, alignment isconsiderably less critical. The preferred surfaces for reflectors 10 and12, therefore, are curved. The most preferred surface shape is aspherical surface.

It is known from laser and beam wave-guide technology that the axialspacing of two reflectors can be as large as, but not greater than,twice the radius of curvature of the reflectors and stable resonancewill still be achieved. However, it has been found that the beam waistas shown in FIG. 1 becomes very narrow at larger spacings, therebyrendering a portion of the volume between reflectors insensitive tointrusion. In order to insure that essentially the entire volume betweenreflectors is sensitive to intrusion the preferred ratio R/d of theradius of curvature R and the spacing d is between about 1.5 and about2.0. Although R/d can range from 0.5 to infinity, less than about 1.5causes the beam waist to be narrow, and greater than about 2.0 resultsin a very close spacing of the reflectors.

In general, a resonant cavity resonates in many modes. Each mode ischaracterized by a certain geometric distribution of energy and acertain resonant frequency, the so-called eigenfunction and eigenvalue,respectively, which comprise a possible solution to the wave equation.In principle, any arbitrary distribution of energy is possible bycombining the individual modes in a suitable way. In practice, however,it is difficult to excite the cavity with just the right amount of eachmode. For that reason, the preferred embodiments of the presentinvention utilize a single mode of resonance which has the desiredgeometrical shape.

FIGS. 2a and 2b show front and cross-sectional side views of rectangularshaped reflectors which can be used to produce an essentially planarpattern of resonant energy. Rectangular reflector 20 includes inputcoupling iris 21 and output coupling iris 22. Energy from the energysource enters the resonant cavity through input coupling iris 21. Outputcoupling iris 22, which alternatively may be in the other rectangularreflector, allows energy to pass from the resonant cavity to thedetector.

When electromagnetic energy is used, the mode generated in the resonantcavity is termed the transverse electromagnetic TEM_(qmn) mode, where q,m, n are integers denoting the number of intensity minima in the axialand two transverse directions, respectively. Of course, acoustic wavesare longitudinal, not transverse; however, they obey the samedifferential equation and, in this case, satisfy the same boundarycondition, so that the mathematical form of the acoustic solution isidentical to that of the electromagnetic one. Therefore, the sameTEM_(qmn) designation is used for the acoustic solution, it beingunderstood that this is not a description of the physical nature of thewave.

When the reflectors are rectangular, the distribution of energy in themidplane of the cavity is approximately,

    [H.sub.n (x√2/w)].sup.2 (w.sub.0 /w).sup.2 [H.sub.n (y√2/w)].sup.2 e-2(x.sup.2 +y.sup.2)/w.sup.2       Equation 1

where x and y are rectangular coordinates, w is a length parameterdepending on reflector geometry and the axial distance between thereflectors, w₀ is the value of w at the beam waist and H_(n) and H_(m)are Hermite polynomials of order m and n respectively The integers m andn determine the particular mode, which is denoted as TEM_(mn).

One particularly useful mode is the rectangular TEM_(m0) mode. This modecan result in a planar distribution of resonant energy. Equation 1 canbe simplified when n = 0 since H₀ = 1. The resulting energy distributionis described as

    (w.sub.0 /w).sup.2 [H.sub.m (x√2/w)].sup.2 e-2(x.sup.2 +y.sup.2)/w.sup.2                                         Equation 2

FIG. 3a shows the distribution of energy in a resonant cavity formed byrectangular reflectors 20 and 23 for the rectangular TEM₈₀ mode. FIG. 3bshows the energy distribution at the beam waist. It can be seen thatthis energy distribution forms essentially a planar curtain of resonantenergy. The curtain is formed by a plurality of energy "bundles" whichare arranged side by side. The number of energy bundles equals m+1. Inmost applications, it is desirable to make m as large as possible. Theterm "planar" is used throughout to described an energy curtain which,in its narrow dimension, is one "bundle" thick. This is approximatelyequal to √λd, where λ is the wavelength and d the reflector spacing.

FIGS. 4a and 4b show a punch press 30 including machine operator safetyapparatus of the present invention. A resonant cavity formed byrectangular reflectors 32 and 34 is mounted between the operator and thehazardous area (ram 36 and die 38) of punch press 30. Other means ofingress to the hazardous area have been blocked off so that the operatormust insert his hand in and through the resonant cavity in order to getto the hazardous area of the punch press 30.

Rectangular reflectors 32 and 34 are generally similar to reflectors 20and 23 shown in FIG. 3. Energy from energy source 40 is coupled into theresonant cavity through input coupling iris 42 in reflector 32. Energyis coupled out of the resonant cavity to detector 44 through outputcoupling iris 46 in reflector 32. The output of detector 44 is directedto machine control 48, which controls the operation of punch press 30.

A planar distribution of resonant energy is produced in the resonantcavity. In one successful embodiment, the energy was electromagneticenergy produced by a microwave source. The particular resonant modeutilized was the rectangular TEM_(mO) mode, where m = 10. In thisembodiment, detector 44 was a microwave detector.

In operation, punch press 30 operates as normal as long as the resonantcondition is present in the resonant cavity. Any disturbance in theresonant condition is sensed by detector 44. Control means 48 stopspunch press 30 whenever detector 44 senses a disturbance in the resonantcondition. For example, whenever a machine operator attempts to reachinto the hazardous area of punch press 30, his hand 50 destroys theresonance in the resonant cavity. This is sensed by detector 44 andcontrol means 48 turns the punch press 30 off. As soon as the operator'shand 50 is removed from the resonant cavity and resonance is againestablished, detector 44 senses the resonant energy and machine control48 starts punch press 30. Manual reset is also possible, and in somecases is preferable.

The system shown in FIGS. 4a and 4b has several advantages. First, itrequires relatively few parts. Second, the apparatus requires no movingparts. Third, the energy used for the proximity sensing is confined tothe specific spatial distribution of the beam within the resonantcavity. Movement outside of the resonant cavity by either the machineoperator or the machine does not disturb resonance and does not,therefore, cause the machine to be stopped. Fourth, machine control 48allows the machine to operate only when detector 44 indicates thatenergy is resonating in the cavity. If the safety apparatus is in someway disabled, machine control 48 will not allow the machine to operate.Failure of either energy source 40 or detector 44, or failure toestablish resonance because of some change in the resonant cavity,causes the machine to remain shut down until the safety apparatus isrepaired.

A second resonant energy distribution which has several usefulapplications is a cylindrical annular distribution. This distribution isproduced by circular reflectors. FIGS. 5a and 5b show front and sideviews of a circular reflector which can be used to generate cylindricalannular resonant modes. Reflector 60 is circular with an essentiallyspherical surface. Input iris 62 couples energy into the resonantcavity, and output iris 64 couples energy from the cavity to a detector.

It should be noted that input iris 62 is not located at the center ofreflector 60, but rather is located near the periphery. The energy willbe introduced into the resonant cavity, therefore, at a location not onthe axis defined by a line connecting the centers of curvature of thetwo reflectors. It has been found that to generate a cylindrical annulardistribution of energy, in which the resonant energy is at a maximumnear the periphery of the reflectors and at a minimum (essentially zero)on the axis, the energy must be introduced off axis. Further descriptionof "off-axis" excitation of high order beam modes is contained in mypreviously mentioned co-pending patent application entitled "High OrderBeam Mode Resonator".

When the reflectors of the resonant cavity are circular, thedistribution of energy between the reflectors can be expressedapproximately as

    (w.sub.O /w).sup.2 (2r.sup.2 /w.sup.2).sup.l [L.sub.p.sup.l (2r.sup.2 /w.sup.2)].sup.2 [e.sup.-2r.spsp.2/.sup.w.spsp.2 ] cos.sup.2 lφEquation 3

where r is the radius, φ is the azimutual angle of the cylindricalcoordinate system, and w is a length parameter depending on the mirrorgeometry and axial position between the mirrors. The integers p and ldetermine the particular mode, which is denoted as TEM_(p) l. L_(p) ^(l)is the generalized Laguerre polynomial.

In the case of a cylindrical annular resonant mode, p is zero. In thiscase, Equation 3 is simplified, since L_(O) ^(l) = 1. Equation 3 thenbecomes

    (2r.sup.2 /w.sup.2).sup.l [e.sup.<2r.spsp.2.sup./w.spsp.2 ] cos.sup.2 lφ (w.sub.O /w).sup.2                                 Equation 4

FIG. 6 shows the energy distribution at the beam waist for thecylindrical TEM_(O8) mode. The energy is distributed in 2l energybundles over an annulus whose radius is w(√l/2). It is advantageous,therefore, to make l as large as possible. Cylindrical annular modeshaving l as high as 25 have been successfully produced.

Since the energy in the cylindrical annular mode is confined to anannulus, the entire center portion of the reflector can be removedwithout affecting the resonance. A machine tool can be located along theresonator axis without disturbing the resonant condition.

FIG. 7 shows a partial cross-sectional view of such an embodiment. Aresonant cavity is formed by circular reflectors 80 and 82. Energysource 84 provides energy to the resonant cavity. This energy isintroduced off axis to generate a cylindrical annular resonant mode.Detector 86 senses the energy in the resonant cavity.

The reflectors 80 and 82 have their center portions removed so that aram 90 and die 92 may be located along the axis of the resonant cavity.Since the energy is confined to an annulus surrounding the ram 90 anddie 92, the machine operator cannot insert his hands into the dangerousarea without disturbing the resonant energy. Machine control 94 controlsthe punch as a function of the signal from detector 86. Wheneverresonance is disturbed, punch 90 is stopped by machine control 94.

The present invention, although particularly advantageous for machineoperator safety, can be used in any application where it is desired tosense the ingress to or egress from a particular area. A resonant cavityis used to confine the beam energy in a resonant condition to a locationof ingress to or egress from the particular area. Any change in theresonant condition will indicate that some object is entering or leavingthe protected area. The particular resonant mode selected will dependupon the nature and shape of the area to be protected.

Among the applications for the present application in addition tomachine operator safety systems is for use as intrusion detectors andparts counters. In the intrusion detector application, any object whichpasses through the resonant cavity on its way to the protected area willdestroy resonance. This change in the resonant condition can be detectedand the signal from the detector can be used to sound an alarm or otherform of warning device.

The present invention also may be used as a parts counter where partsare being ejected from an automatic machine tool. The resonant cavity islocated so that the ejected parts have to pass through the resonantcavity. Each time a part is ejected, it causes a disturbance in theresonant condition which is detected. The signal from the detector isthen directed to a counting device which counts the number of times thatthe resonant condition is disturbed. This total corresponds to thenumber of parts which have been ejected. Such a device also verifiesthat the part has ejected from the die, thereby allowing the press tocycle again.

In conclusion, the present invention is a proximity sensor having a widevariety of applications. It has a minimum number of parts and no movingparts. A variety of resonant modes having different spatialdistributions may be used depending upon the particular applicationdesired.

The present invention has been described with reference to a series ofpreferred embodiments. It will be understood, however, by workersskilled in the art that changes in form and detail may be made withoutdeparting from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:
 1. Apparatus for protecting a machineoperator from a hazardous area of a machine, the apparatuscomprising:means for generating a spatial distribution of resonantenergy interposed between the operator and the hazardous area whereinthe means for generating a spatial distribution of resonant energycomprises,beam resonant cavity means wherein the resonant beam cavitymeans comprises concave curved surfaces facing one another and separatedfrom one another along an axis, and energy source means for introducingenergy into the beam resonant cavity means, the energy being resonant ina mathematically describable spatial distribution wherein the energysource means introduces energy into the resonant cavity means at aposition not on the axis; detector means for sensing energy within thespatial distribution; and control means for controlling the machine inresponse to the detector means.
 2. The apparatus of claim 1 wherein theconcave curved surfaces have substantially spherical surfaces.
 3. Theapparatus of claim 1 wherein the ratio of the radius of curvature of thereflectors to the axial spacing of the reflectors is between 1.5 and2.0.
 4. The apparatus of claim 1 wherein the spatial distribution ofresonant energy is a substantially cylindrical annular distribution. 5.The apparatus of claim 1 wherein the spatial distribution of resonantenergy is a substantially planar distribution.
 6. The apparatus of claim1 wherein the resonant energy is electromagnetic energy.
 7. Theapparatus of claim 1 wherein the resonant energy is acoustic energy. 8.The apparatus of claim 1 wherein the presence of an object in thespatial distribution of resonant energy causes a disturbance in thespatial distribution.
 9. The apparatus of claim 8 wherein the controlmeans causes the machine to cease operation when a disturbance in thespatial distribution occurs.